Night vision systems include image intensification, thermal imaging, and fusion monoculars, binoculars, and goggles, whether hand-held, weapon mounted, or helmet mounted. Image intensification systems are typically equipped with one or more image intensifier tubes to allow an operator to see visible wavelengths of radiation (approximately 400 nm to approximately 900 nm). They work by collecting the tiny amounts of light, including the lower portion of the infrared light spectrum, that are present but may be imperceptible to our eyes, and amplifying it to the point that an operator can easily observe the image. These systems have been used by soldier and law enforcement personnel to see in low light conditions, for example at night or in caves and darkened buildings. A drawback to image intensification systems is that they may be attenuated by smoke and heavy sand storms and may not see a person hidden under camouflage.
Thermal imagers allow an operator to see people and objects because they emit thermal energy. These systems operate by capturing the upper portion of the infrared light spectrum (approximately 7000 nm to approximately 14,000 nm), which is emitted as heat by objects instead of simply reflected as light. Hotter objects, such as warm bodies, emit more of this wavelength than cooler objects like trees or buildings. Since the primary source of infrared radiation is heat or thermal radiation, any object that has a temperature radiates in the infrared. One advantage of thermal imagers is that they are less attenuated by smoke and dust and a drawback is that they typically do not have sufficient resolution and sensitivity to provide acceptable imagery of the scene.
Fusion systems have been developed that combine image intensification with a thermal sensor (approximately 7,000 nm to approximately 14,000 nm) in a single enclosure. The image intensification information and the thermal information are fused together to provide an image that provides benefits over just image intensification or just thermal imaging. Whereas image intensifiers can only see visible wavelengths of radiation, the fused system provides additional information by providing long wave information to the operator.
FIG. 1 is a block diagram of an image intensifier system 100 capable of viewing a target or area of interest 108. The electronics and optics are housed in a housing 102, which can be mounted to a military helmet through a mount 128, and are powered by a power source 144. Information from a first image intensification (I2) channel 120A and a second I2 channel 120B is directed to an operator through one or more eyepieces 106 located in an end portion 132A, 132B. The eyepieces 106 have one or more ocular lenses for magnifying and/or focusing the intensified image. The I2 channels 120A, 120B are configured to process information in a first range of wavelengths (the visible portion of the electromagnetic spectrum from approximately 400 nm to approximately 900 nm). The I2 channels 120A, 120B, located in an end portion 130A, 130B of the housing 102, may have an I2 tube 122 and an objective with adjustable focus 124. The housing 102 has two actuators coupled to a power supply 142. The on/off actuator 160 allows the operator to turn the system on and off and the I2 channel gain actuator 162 allows the operator to adjust the gain of the I2 tubes 122.
FIG. 1B is a block diagram of an image intensifier system with a thermal camera and a separate display strapped thereto to provide a picture-in-a-picture view of a scene. The image intensifier system 100 may be the system shown in FIG. 1. A camera 226, for example the Alpha™ camera from Indigo Systems of Goleta, Calif., may output an analog RS170 video signal to a display 234′, for example a 640×480 display as incorporated in the CV-3 Video Viewer from Micro Optical of Westwood, Mass. The camera 226 and the display 234′ are powered by separate power supplies. A light turning element 232′ may be disposed in front of the objective lens 124 of the first channel 120B to allow the visible presentation of the thermal image to be injected into the I2 channel and thereby viewable through the eyepiece 106. The position, size, and focus of the virtual thermal image is affected by the position of the light turning element with respect to the objective lens thereby preventing image fusion or simultaneous view of the thermal and image intensified scenes through the eyepiece.
In a thermal imager, the scene data may be sensed by a two-dimensional array of infrared-detector elements. The detector elements can create a very detailed temperature pattern, which can then be translated into electric impulses that are communicated to a signal-processing unit. The signal-processing unit may then translate the information into data for a display aligned with an eyepiece. Thermal imagers can sense temperatures with range in excess of −40 to +50° C. and can detect changes in temperature as small as 0.025° C. The different temperatures are typically displayed as varying shades between black and white. The display may also display system information as well as scene information.
In a fusion system, the display may be aligned with an image combiner for viewing through one of the eyepieces. Fusion systems typically have the optical axis of the thermal channel physically offset a fixed distance from the optical axis of the I2 channel. The fusion system is typically factory aligned such that the image from the thermal channel is fused and is aligned in the eyepiece with the image from the I2 channel when the image being viewed is at a predetermined distance, often aligned at infinity. At distances different from the predetermined distance, parallax can cause a misalignment of the two images in the eyepiece. The parallax problem exists if the thermal channel and the I2 channels are offset in the horizontal as well as the vertical directions.